Radio identification (RFID) systems have been widely used in recent years. Typical examples of RFID systems include systems that use electromagnetic waves equivalent to a UHF band (900 MHz band) or microwaves (2.45 GHz) as communication media, and systems that use mutual induction magnetic fields. Among such systems, RFID systems that use electromagnetic waves in the UHF band have attracted much attention because these RFID systems have relatively long distances over which communication is possible.
As antennas that may be used in order for a tag reader to communicate with radio frequency identification tags using UHF-band electromagnetic waves, microstrip antennas in which a microstrip line is utilized as an antenna have been proposed (see Japanese Laid-open Patent Publication No. 4-287410 and Japanese Laid-open Patent Publication No. 2007-306438). Note that the radio frequency identification tag will be referred to as an “RFID tag” hereinafter for the sake of explanatory convenience.